Silicon photonics technology is a key to the future ultra fast optical electronics and has been frequently reported in recent years. Silicon photonics technology is based on silicon materials, which had been considered to be unsuitable for optical applications. Silicon photonics technology includes, for example, a silicon Raman laser which oscillates by photoexcitation and a silicon optical modulation device which performs an ultra fast GHz operation.
In the silicon photonics, optical interconnections are formed on a chip by a CMOS compatible LSI technology. The optical interconnects have functions of emission, transmission and reception and includes individual optical electronics elements. Silicon based optical modulation devices, switches, waveguides and light-receiving devices have been actively investigated primarily in the United States. However, silicon based current injection type light-emitting devices have not been successfully developed as practical devices.
A current injection type silicon light-emitting device functions as a “transmitter” in the optical interconnection and is considered as a key semiconductor light source. Current injection type silicon light-emitting devices are highly advantageous from the standpoint of the cost and manufacturing requirements. However, such silicon light-emitting devices are difficult to achieve because silicon (Si) is an indirect band gap semiconductor and is non-radiative because of a phonon-assisted transition.
Lorenzo Pavesi., Materials Today, January 2005 and Philippe M. Faucet., Materials Today, January 2005 disclose that a silicon nano-particle and an ultra-thin silicon film have been proposed and investigated to provide a light-emitting function to silicon, in other words, to achieve the indirect-to-direct-transition modulation in silicon just like a direct band gap compound semiconductor with strong radiation represented by gallium arsenide (GaAs). However, practical silicon based current injection type light-emitting devices have not yet been realized. This has been an unsolved problem for about 20 years.
Both the nano-particle and the ultra-thin film have been considered to be derived from the “quantum confinement effect,” from a view point of a light-emitting mechanism. Accordingly, many research institutes are trying to achieve silicon light-emitting devices base on this effect.
FIG. 1 is a band diagram of crystal silicon and gallium arsenide (GaAs). Gallium arsenide (GaAs) generates a strong interband emission derived from an optically allowed electric dipole transition. There are two conditions in order to realize a strong interband emission. One condition is a selection rule regarding wave numbers. According to the wave number selection rule, the energy gap must be minimized at a specific wave number. The other condition is a selection rule regarding the symmetry of a wave function. According to the symmetry selection rule, when either one of the conduction band and the valence band is even function, the other one is odd function, at the wave number where the gap is minimized.
The symmetry selection rule is described in more detail below. Intensity of emission and light absorption between two energy levels is represented by <upper energy level|transition dipole moment μ|lower energy level>. In the case of a direct band gap compound semiconductor, <s|μ|p>=∫(even×odd×odd)dr≠0, where two energy levels are represented by s-orbital (even function) and p-orbital (odd function) on atomic orbital approximation. This results in a strong emission due to an optically allowed transition. On the other hand, in the case of indirect band gap semiconductor, <p|μ|p>=∫(odd×odd×odd)dr=0, where both two energy levels are represented by p-orbital. This results in non-radiative emission due to an optically forbidden transition.
Gallium arsenide (GaAs) has the minimum gap at the Γ point and satisfies the wave number selection rule. Further, wave functions of the conduction band and the valence band are represented by s-orbital and p-orbital respectively as described above, and therefore gallium arsenide (GaAs) satisfies the symmetry selection rule.
Noting the Si nano-particles and the ultra-thin Si films, the conduction band of silicon, which is located in the vicinity of the X point due to the quantum confinement effect, moves to the Γ point. The conduction band provides a direct transition so that the gap is minimized at the Γ point. The band shape is changed to satisfy the wave number selection rule due to the quantum confinement effect.
Considering the symmetry of a wave function, a wave function of the valence band is p-orbital and that of the conduction is also p-orbital. The symmetry of a wave function does not change even when the quantum confinement effect occurs. In other word, the symmetry selection rule is not satisfied.
It has been understood that the quantum confinement effect provides pseudo-direct transition in silicon. The quantum confinement effect may not cause a strong emission from silicon. Therefore, it has been understood that conventional silicon light-emitting devices do not have desired characteristics and are expected to be difficult to realize silicon light-emitting devices. This is indicated in FIG. 2, illustrating a contour drawing of internal quantum efficiency η of photoluminescence.
The vertical axis represents a radiative lifetime τr, the horizontal axis represents a non-radiative lifetime τnr, and the dotted line represents the contour drawing of the quantum efficiency. The reference crystal silicon and the direct gap compound semiconductor are oppositely positioned at η˜0 and η˜1 respectively. The lifetime r and τnr are an estimated value from literature.
As shown in FIG. 2, the quantum efficiency is 0 at the bottom-left corner and 1 at the upper-right corner. The efficiency drastically changes between 0 and 1 on the diagonal line connecting the bottom-left corner and the upper-right corner. This is because τr is long, and τr is strongly affected non-radiative recombination at the surface and defects. A nano-particle may show an efficient photoluminescence at the present time. However, even this nano-particle has τr of about micro seconds, and therefore has a very long radiative lifetime compared with a compound semiconductor.
Therefore, to realize silicon mission devices, it is desired to drastically improve the quantum efficiency so as not to be affected by the non-radiative recombination. In order to realize this, it is desired to introduce two energy levels connected with an optically allowed transition within the silicon band gap. These two energy levels may generate a strong emission, which is as strong as the compound semiconductor. However, in the conventional band modulation technologies such as the nano-particle and the ultra-thin film, the band modulation effect is limited, and therefore it is difficult to create the allowed transitions in silicon.
Meanwhile, there is a research using an active layer doped with specific impurity atoms. T. G. Brown and D. G. Hall., Appl. Phys. Lett. Vol. 45, No. 5 (1986) discloses that sulfur-doped (S-doped) crystal silicon was found by Brown at University of Rochester in 1986 and it has been known that the S-doped crystal silicon has an exceptional strong emission. However, the S-doped crystal silicon generates a strong emission only at low temperature. There is a problem in that the S-doped crystal silicon quenches at room temperature as shown in the temperature dependency of the photoluminescence (PL) of FIG. 5.
As described, practicable silicon-based light-emitting devices have not been realized by the conventional band modulation technologies.